Hi, I couldn't find any existing incident resembling my situation, so here goes. I have conventional gas heat/forced hot air heat system (along with ac using the same air handler). I replaced my old programmable thermostat (run on batteries) with new honeywell RET97E5D1005/U Wi-Fi thermostat. I researched all about the need of the C-wire. Fortunately my wires to the thermostat has two extra unused wires. Using one of the extra unused wires to connect to Common at the air handler, i was able to power up my new thermostat fine. Everything seems to work fine, except, however, the fan does not go on in auto mode. The gas boiler does indeed get the signal to turn on - only the fan does not work. If i turn the fan on in manual mode, the fan works. I called Honeywell support, and they are saying there is nothing wrong with the thermostat ( I tend to agree since i purchased two new thermostats and both have same symptoms for this particular zone - i find it unlikely that both have the same defect.. but then again.. who knows..). If i uninstall the new thermostat, and install my old one, the auto mode works fine. If i measure voltage drop across RH(R) wire and W (as well as from RH to C), I get 19-20 volts (instead of expected 24volts). Could that be the cause of my issue? could the current drop to the blower be too much when adding the new thermostat into the mix? Any suggestions? Unfortunately, I'm not exactly sure the make of my air handler ("janitrol" sounds familiar.. but not sure..), but attached are some photos of the schematic. thanks for your help!

I purchased the Honeywell thermostat model RTH8580WF. My boiler is equipped with a Honeywell Aquastat L7224 controller. It is clear that I attach the RH and W lines of the thermostat to T terminals. It is not clear where I attach the C common line to power the thermostat. Please advise.